I don't like getting into these threads because I always end up arguing my points rather than just discussing them, however since you're asking what you should buy, i'll add an unbiased input.
A lot of people will tell you that the Wii is only for casuals, it's an urban myth, it IS fair to say the Wii has more crappy games than the other 2 by far, but Wii does have some good ones. It's also noteworthy that there's LOADS of casual games on the online playstation store (albeit they're about £4-£6 each) and with natal hitting 360 soon there'll no doubt be a load of casual stuff on there too. So really, if you want casual, all 3 can offer you something.
In terms of controllers, it saddens me to say, but PS3 has the worst controller. It's not a bad controller, but the 360 one just feels, i dunno, easier to hold? And Wii's main selling point is it's controller, which i think pretty much makes it incomparable to the other 2.
When it comes to exclusive games, i think PS3 wins hands down (its the main reason i got mine) Uncharted 2, MGS4, Killzone 2 (its a good game it just feels really heavy) LittleBigPlanet, Ratchet & Clank series, SingStar, Gran Turismo, Wip3out, SOCOM etc, there really is something for everyone on PS3. 360 does offer fun stuff like L4D and Gears (and I guess Halo tho i really dont think Halo is anything special) and PGR, it doesnt really have a such a large variety of great exclusives as PS3.
For online, the Wii is the clear loser, it uses a haphazard friend code system, it's awkward to set up and it's laggy. XBL probably is the best (however they charge you for it) and it has the best tech support by miles. PSN is pretty good, it's not got as many bells and whistles as XBL, and the support staff are kinda rubbish, but it's free, it works fine and it's constantly getting better with each firmware update.
In terms of gameplay I don't really think there's a clear victor. Wii has it's own style and there is honestly little difference between PS3 and 360 in terms of gameplay.
Looking at graphics, PS3 and 360 are pretty much tied at top. Some 360 games look better than most ps3 games, some ps3 games look better than most 360 games etc. The main thing is both run on 720p and both can put out 1080p.
For extra features I'd say PS3 wins, it has a capable browser, video, BBC iPlayer, Music, Pictures, PlayTV, VidZone, customizable, animated themes that change with the time of day, HOME etc. It also has DVD and Blu Ray which is awesome if you like films.
In terms of cost, I think that all are pretty neck to neck. PS3 is a bit more pricey but like Theo said, you really get your money's worth. Built in WiFi, Blu Ray, rechargable controllers, Blue tooth support etc. With the Wii, it's pretty average for it's price, again you get what you pay for. With the 360, it seems cheapest at first, but to utilize it it'll cost you a fair bit more in the long run.
Wow, kinda tl;dr there. Anyway I hope the above points help clear things up a little for you, but at the end of the day, no one can really suggest what you should get without knowing what you like.